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Thermodynamics


Behzad NJ

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Hello guys

Yesterday I was studying thermodynamics , now I've got this problem !

In the power plants we want to convert Q to W , in most cases at the end of cycle we uses some tools to reduce the amount of this Q , but why ? Why don't we use low amount of Q ? so that we don't need to reduce it at the end .

And could you please let me know what books do you study in thermodynamics ?

I'm studying Thermodynamics by Cengel .

 

 

 

Best Regards

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Q is heat, which is energy being transferred and not a substance. So you can talk about converting heat into work, but technically you do not lower the Q of the coolant; what you do is lower the temperature. In a nuclear reactor (they types I'm familiar with, at least) this is to condense the steam back into water so you can pump it back to the heat exchanger. Perhaps you have a specific example in mind to which you could post a link or upload a diagram?

 

If the Q was low you couldn't do much work, since Q<W

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Sounds like you're asking why heat must be rejected at the end of the cycle, which will be explained when you get to the Second Law.

It's not possible to use all of the available heat to do work, and what you can't use you have to reject to a colder body.

 

I still have my copy of Cengel and Boyle's thermo text.

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The above answers are both correct. It's because of the 2nd law of thermodynamics.

 

But in much simpler words:

For example, first you had 1 kg of steam (which is essentially water), and it was at 160 C, in the vapor phase (approx.; 6 bars). If you had a steam engine, you could do some real work with that.

 

But what if I would give you the equivalent amount of heat in a different form? What if I would say I give you 650 kg of water at 21 degrees (when the outside temperature is 20). It's just a large bucket full of lukewarm water. It's not gonna do much for you.

 

After you used your material to do some work, it will not be at ambient temperature. It will be warmer than ambient, but it will be completely useless. Sure, with some smart contraptions you might extract a little more - but usually the efficiency of such contraptions is low and they tend to be expensive.

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  • 1 month later...

The Rankine cycle requires that the turbine exhaust steam be condensed back to liquid water, which means that some form of heat rejection is necessary. Conventionally, this is done by wet cooling, which circulates cooling water through tubes in contact with the exhaust steam. The cooling water is then dripped down an updraft in a large cooling tower, resulting in evaporative cooling of the cooling water. Making the exhaust steam do work is another way of reducing its enthalpy and getting it to condense. See http://www.freepatentsonline.com/7987677.pdf

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After you used your material to do some work, it will not be at ambient temperature. It will be warmer than ambient, but it will be completely useless. Sure, with some smart contraptions you might extract a little more - but usually the efficiency of such contraptions is low and they tend to be expensive.

 

If I understand what you mean here (the bolded part) this was actually the idea behind the compound steam engine. High pressure steam was used to drive one set of pistons, and the low pressure exhaust steam was used to drive a second set extracting yet more energy from the steam before it was finally released The second set of pistons, however, always produced less energy than the first set, because of the loss of energy into work from the first set.

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If I understand what you mean here (the bolded part) this was actually the idea behind the compound steam engine. High pressure steam was used to drive one set of pistons, and the low pressure exhaust steam was used to drive a second set extracting yet more energy from the steam before it was finally released The second set of pistons, however, always produced less energy than the first set, because of the loss of energy into work from the first set.

 

No. A compound steam engine, or a triple or quadruple expansion steam engine, instead of expanding the steam from P1 to P2 in one cylinder, distriibutes the expansion across two, three or four cylinders. The total expansion is the same as if a single cylinder was used, but the energy is extracted more efficiently, leading to more work from the same steam flow. The reason for the better efficiency is to do with the temperature difference between inlet and exhaust, if this is too large there can be condensation in the cylinder when the inlet steam enters the previously exhausted volume, leading to waste of steam, and there are other practical factors such as valve timing and mechanical design. If you try to expand steam from say 50 bar to 1 bar in one cylinder, the admission valve would have to close after 2% of the stroke, which is difficult. The cylinder would have to be both large enough for the exhaust flow at 1 bar, and also strong enough for the inlet pressure of 50 bar. This is not an optimum mechanical design.

More work can be extracted from steam at higher pressure, and expansion from higher pressure is most efficiently done in several stages (as it is in a turbine).

 

Paul

 

The engine designer would aim to get equal power from each cylinder, not (as you say) always more from the HP cylinder.

Sometimes in large engines the steam was reheated between one cylinder and the next, using waste heat, and improving efficiency.

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No. A compound steam engine, or a triple or quadruple expansion steam engine, instead of expanding the steam from P1 to P2 in one cylinder, distriibutes the expansion across two, three or four cylinders. The total expansion is the same as if a single cylinder was used, but the energy is extracted more efficiently, leading to more work from the same steam flow. The reason for the better efficiency is to do with the temperature difference between inlet and exhaust, if this is too large there can be condensation in the cylinder when the inlet steam enters the previously exhausted volume, leading to waste of steam, and there are other practical factors such as valve timing and mechanical design. If you try to expand steam from say 50 bar to 1 bar in one cylinder, the admission valve would have to close after 2% of the stroke, which is difficult. The cylinder would have to be both large enough for the exhaust flow at 1 bar, and also strong enough for the inlet pressure of 50 bar. This is not an optimum mechanical design.

More work can be extracted from steam at higher pressure, and expansion from higher pressure is most efficiently done in several stages (as it is in a turbine).

 

Paul

 

The engine designer would aim to get equal power from each cylinder, not (as you say) always more from the HP cylinder.

Sometimes in large engines the steam was reheated between one cylinder and the next, using waste heat, and improving efficiency.

 

Ahh. Ok, I understand what you're getting at. Thank you for correcting my misconceptions.

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